The present invention relates to integrated circuits, and more particularly, to a method for bonding wafers together to form integrated circuits having a stack of thin layers.
Modern integrated circuits are typically constructed in a thin layer in a semiconducting layer on a substrate wafer such as silicon. This essentially two-dimensional structure limits both the size of the integrated circuit and the speed at which the circuit operates. The speed at which an integrated circuit operates is determined by the distance between the farthest separated components that must communicate with one another on the chip. For any given number of components, the path lengths will, in general, be significantly reduced if the circuit can be laid out as a three dimensional structure consisting of a number of vertically-stacked layers of circuitry, provided the vertical distances between the layers are much smaller than the width of the chips that make up the individual layers.
One promising scheme for providing such stacked structures utilizes a method for stacking and bonding entire wafers. In this method, integrated circuits are fabricated on conventional wafers. Two wafers are bonded vertically by thinning one wafer in a first coarse thinning operation by removing material from the back of the wafer. The circuitry on the front surface of each wafer is covered with an insulating layer having metal filled vias that make contact with the underlying circuitry and act as electrical connection points between the two wafers. In addition, the front surfaces of the wafers include bonding pads that are planar metal areas that do not connect to the underlying circuitry. The bonding pads are provided to increase the bonded area. The front surfaces of the wafers are then placed in contact with one another so that the bonding pads on one wafer are in contact with the pads on the other wafer. Thermal diffusion bonding is then used to bond the metal pads, and hence, the wafers together. One of the wafers is then further thinned to a thickness of a few microns by etching or mechanically grinding the back surface of that wafer further. Once the wafer has been thinned, a new set of vias is opened in the backside and filled with metal to provide the connection points to the pads on the front side of the wafer that make connections with the circuitry in the wafer. In addition, a new set of bonding pads is formed on the backside of the wafer so that another wafer can be bonded to the stack. The process is then repeated until the desired number of layers has been bonded to form the three-dimensional stack. The three-dimensional stack is then cut into three-dimensional chips and packaged.
Each time a top wafer in the stack is thinned, the wafers below that wafer are subjected to a significant amount of lateral stress by the grinding process. Accordingly, the bonds that hold the wafers together must withstand these large stresses. In principle, the ability of the wafers to withstand these stresses can be increased by increasing the bond area, i.e., devoting more area to bonding pads between the wafers. Unfortunately, this approach has the drawback of reducing the number of vertical conductors that can be provided in circuits requiring a high density of inter-layer vertical conductors.
This process also requires a considerable amount of xe2x80x9cbackside processingxe2x80x9d to provide the new set of contacts on the backside of the wafer. The generation of the vertical conductors requires a number of masking and deposition steps. Each conductor is structurally similar to a golf tee. The xe2x80x9cheadxe2x80x9d of the tee provides the area needed to accommodate alignment errors. Separate mask and deposition steps are required to etch the via that forms the stem of the tee and the head that sits on this stem. The via must also be lined to prevent diffusion of the metal into the surrounding silicon. Such extensive backside processing can substantially reduce the yield of stacked wafers.
The situation is made more complicated by the lack of fiduciary marks on the backside of the thinned wafer. Hence, precise alignment of the masks that define the locations of the vias with respect to the circuitry on the front side of the wafer is difficult. Misalignment of these vias leads to defects that render the entire stack of chips useless. In addition, the semiconductor processing steps required by the backside processing of the top layer subject the underlying layers to thermal and mechanical stresses that are repeated with each new layer.
Broadly, it is the object of the present invention to provide an improved method for stacking and thinning wafers to generate a three-dimensional integrated circuit.
It is a further object of the present invention to provide a method that eliminates the backside processing steps utilized in prior art stacking techniques.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.
An integrated circuit wafer element and an improved method for bonding the same to produce a stacked integrated circuit. An integrated circuit wafer according to the present invention includes a substrate having first and second surfaces constructed from a wafer material, the first surface having a circuit layer that includes integrated circuit elements constructed thereon. A plurality of vias extend from the first surface through the circuit layer and terminate in the substrate at a first distance from the first surface. The vias include a stop layer located in the bottom of each via constructed from a stop material that is more resistant to chemical/mechanical polishing (CMP) than the wafer material. The vias may be filled with an electrically conducting material to provide vertical connections between the various circuit layers in a stacked integrated circuit. In this case, the electrical conducting vias may also be connected to various circuit elements by metallic conductors disposed in a dielectric layer that covers the circuit layer. A plurality of bonding pads are provided on one surface of the integrated circuit wafer. These pads may be part of the conducting plugs resulting from filling the vias with the conducting material. These pads preferably extend above the surface of the integrated circuit wafer. A stacked integrated circuit according to the present invention is constructed by bonding two integrated circuit wafers together utilizing the bonding pads. One of the integrated circuit wafers is then thinned to a predetermined thickness determined by the depth of the vias by chemical/mechanical polishing (CMP) of the surface of that integrated circuit wafer that is not bonded to the other integrated circuit wafer, the stop layer in the vias preventing the CMP from removing wafer material that is within the first distance from the first surface of the substrate of the wafer being thinned. The thinning operation leaves a portion of the plug in the via standing above the surface of the thinned substrate. A third wafer can then be bonded to the raised portion of the plug by bonding the raised portion of the plug to pads on the third wafer""s surface. The bonding pads on the third wafer surface preferably include a depressed region for engaging the raised portion of the plugs on the second wafer.